1. Field of the Invention
The invention relates to a pressure plate configuration for a motor vehicle friction clutch and in particular to a configuration of pressure plate friction engagement surfaces.
2. The Prior Art
A motor vehicle friction clutch assembly commonly includes a clutch cover fastened to a flywheel, a pressure plate rotatably fixed yet axially displaceable relative to the clutch cover, and a prestressed apply spring acting against the cover to bias the pressure plate toward the flywheel. The apply spring may take the form of a diaphragm spring disposed between the cover and the pressure plate. A driven disc is axially disposed between the pressure plate and the flywheel, and is slidably disposed over the splines of the transmission input shaft. The apply spring forces the pressure plate toward the flywheel, compressing the driven disc therebetween. The frictional linings of the driven disc are engaged by the engagement surfaces of the pressure plate and the flywheel. In the engaged condition, the clutch prevents relative rotation between the engine crankshaft to which the flywheel is fixed, and the transmission input shaft, communicating the engine torque therethrough. A release bearing is used to overcome the apply spring load against the pressure plate to achieve clutch release. In the clutch released condition, the engine is able to rotate independently of the transmission input shaft, as is commonly required when selecting transmission gears. The release bearing is typically displaced to the release position by a pivotable release bearing fork which is connected through a mechanical or hydraulic linkage to an operator displaced foot pedal.
Clutches are selected for particular applications based largely on the magnitude of torque they can sustain before slipping, or the torque capacity of the clutch. For any given clutch, it is generally desired to maximize its torque capacity, without negatively affecting other important performance characteristics of the clutch, such as resistance to wear, and ease of engagement. Variables which influence the torque capacity of a clutch include:
the pressure plate load or clamp load, which is primarily developed by the apply spring;
the number of friction surfaces;
the coefficient of friction between the engaging friction surfaces; and
the effective torque arm length of the friction surfaces.
Torque capacity is increased by increasing any one of the above variables. There are, however, limits on how much any of these factors can be changed. For example, there is a practical limit to how much pressure plate loads can be increased, at least for clutches which are released through a manual pedal system. Increases in pressure plate loads result in increased release loads, and correspondingly higher pedal loads. Excessively high pedal loads would make it difficult for a vehicle operator to release the clutch.
The number of friction surfaces is increased beyond the two associated with a single driven disc in some heavier duty clutches by providing a second driven disc and an intermediate plate in the clutch assembly. However, it is generally impractical to alter the number of driven discs employed for a given clutch/transmission combination due to packaging constraints such as the length of the transmission input shaft and the length of the bell housing which connects the transmission with the engine block, and the location of the release fork.
Changing the friction material comprising the friction linings to achieve a higher coefficient of friction is a potential option. However, the material chosen must not negatively affect either the engagement quality, nor the wear life of the clutch. These constraints have the effect of substantially limiting increases which can be provided in the coefficient of friction.
Changing the effective torque arm length of the friction surfaces can be accomplished by increasing the diameter of the flywheel, the driven disc, the pressure plate, and the clutch cover. However, such changes are undesirable both from a packaging perspective as well as due to the undesired increased rotational inertia of the clutch. It has been noted that the effective torque arm length of the friction surfaces, particularly at the point of initial engagement, is typically much less than it could be, as the initial engagement occurs over less than the full area of the friction material pads and commonly occurs at a location other than an outer periphery of the friction material. The length of the effective torque arm is a function of the engagement between the driven disc and the engagement surface of the pressure plate. Most commonly, such surfaces are formed flat and normal to the axis of rotation. If the engaging surfaces are perfectly flat and parallel to each other, a nominal effective torque arm length will result. However, variance in the smoothness or flatness surface of the friction linings, and variance in the smoothness or flatness of the engagement surface will cause the length of the torque arm to vary. In one known pressure plate, the engagement surface of the plate nominally tapers 0.002 inches (0.05 mm) on one side from an outer perimeter edge to an inner perimeter edge, to provide a slightly concave shape at nominal. Such a shape does not overcome much of a variance in flatness of the friction material. Further, the amount of taper is allowed to vary from nominal by plus or minus 0.003 inches (0.08 mm). At the outer limit of the permitted taper, the engagement surface would be slightly convex, tending to engage even perfectly flat friction material proximate to the inner periphery of the pressure plate, with the resultant torque arm length being at its minimum. The variation of both the friction material and the engagement surfaces can result in the effective torque arm length being significantly shorter than nominal, at least on initial engagement. In such a circumstance, more clutch slippage than is desirable may occur upon initial clutch engagement, and may even reduce the torque capacity of the fully engaged clutch.
It has been noted that the coefficient of friction between the friction material and the frictional engagement surface is lower when the clutch is new, and increases after several clutch engagements. Fewer engagement cycles are needed to increase the coefficient of friction if the force per unit area is relatively high. Therefore, initial engagement torque capacity could be increased more quickly by initially engaging just a portion of the friction material.
Known clutch construction has been shown to be disadvantageous in connection with maximizing clutch torque capacity in that the torque capacity for a given clutch configuration is less than it potentially could be, particularly for initial engagements of the clutch. It is desired to provide a clutch which maximizes the effective torque arm length at the clutch""s friction engagement interface early in the life of the clutch.
A friction clutch assembly for a motor vehicle includes a clutch cover configured for fastening to a flywheel for rotation therewith about an axis of rotation. A pressure plate is disposed on the clutch cover and is connected to the clutch cover for rotation therewith. The pressure plate is axially displaceable relative to the clutch housing, and has a first side disposed toward the clutch cover. The pressure plate has a frictional engagement surface on a second side opposite the first side. An apply spring is disposed between the clutch housing and the pressure plate in a prestressed condition. The apply spring biases the pressure plate away from the clutch housing. The pressure plate friction engagement surface has an annular initial engagement region substantially normal to the axis of rotation extending between a first inner diameter and a first outer diameter. The first outer diameter is proximate to an outer periphery of the pressure plate. The pressure plate also has an annular secondary region radially within the initial engagement region. The annular secondary engagement region defines a concave surface within the pressure plate. The annular second engagement region has a second outer diameter proximate to the first inner diameter, and has a second inner diameter proximate to an inner periphery of the pressure plate.
A pressure plate for a motor vehicle includes an annular disc having a first frictional engagement surface. The first frictional engagement surface has an annular initial engagement region substantially normal to the axis of rotation, and a secondary engagement region disposed radially inwardly of the initial engagement region defining a concave surface within the pressure plate.
The inventive clutch maximizes the effect of torque arm length of the clutch at the clutches friction engagement interface early in the life of the clutch.